Baltimore steel to make Boston traffic a little less harrowing Harbor tunnel prefabricated at BethShip

August 17, 1993|By Peter Jensen | Peter Jensen,Staff Writer

At 12:20 p.m. yesterday, the Alexandra took up the slack in its line and a 7,800-ton behemoth of steel and concrete slipped away on a three-day journey by barge to Boston.

No banners flapped in the breeze. No champagne corks popped. No speeches commemorated the departure of the seagoing tugboat and its cargo from the BethShip shipyard on the Patapsco River. The uneventfulness of the moment was a contrast to the cargo's status as an engineering marvel and its critical importance to the 5.5 million people living in the nation's seventh-largest metropolitan area.

But what you can't see beneath the surface has always been part of making tunnels.

The rust-colored hulk that quietly left Baltimore yesterday was the last of the 12 sections that will make up Boston's Third Harbor Tunnel, a $238 million link from South Boston to Logan Airport that is designed to relieve Boston's traffic-choked downtown.

From its humble beginnings as little more than a heap of iron ore in Sparrows Point, the tunnel has become the key component in the nation's most expensive highway project, a $6.4 billion effort known as the "Big Dig."

As New Englanders can tell you, the Sumner and Callahan tunnels, the city's existing two-lane connections to East Boston, are in gridlock during morning and evening rush hours. The Central Artery, the elevated portion of Interstate 93 that runs through Boston, is similarly paralyzed.

"Without this project, we'd be looking at all-day traffic jams downtown," said Thomas J. Nally, planning director for the Artery Business Community, a business group supporting the project. "This can make things at least marginally easier."

To create the new tunnel, engineers turned 425 miles south to Baltimore and the same technology that created the Fort McHenry tunnel. Like the Interstate 664 tunnel at Hampton Roads in Virginia and a dozen subway tunnels across the country, the Fort McHenry was built with prefabricated immersed tubes.

Unlike traditional tunnels, which are caverns mined deep underground and then lined with concrete, immersed tube tunnels are mostly built in advance and then dropped into an underwater trench.

The technique minimizes the impact on surrounding communities, avoids the surprises that come come from drilling and allows the tunnel to be shallower, eliminating the need for steep ramps at each end.

Under a $60 million contract awarded 21 months ago, Bethlehem Steel Corp.'s BethShip yard at Sparrows Point was assigned to fabricate the project's primary components, the 12 huge steel sections that will form the 3,900-foot-long tunnel.

Each section is about the length of a football field, 322 feet, and is 80 feet wide and 40 feet tall. Each is crafted from 1,500 tons of steel poured and rolled at the adjoining Bethlehem Steel mill.

A 'jigsaw puzzle'

The sections are double-bored like the barrel of a shotgun fitted for a giant. Instead of shells, each bore is big enough to carry two lanes of traffic.

The shipyard welders, fitters, crane operators and carpenters began with much smaller pieces, some only a few feet long. They joined the various parts together to create modules one-eighth the length of a finished section.

"This was like a jigsaw puzzle," said John K. Bass, BethShip's production superintendent for the project. "We started with little pieces and put them together."

It was production on the half-shell, literally. The modules were built in halves that from a distance looked like a cursive "W."

Imagine a shipbuilder bending wooden slats to create a hull. The interior tubes were formed in much the same way.

Sheets of steel were clamped down on a half-moon jig. These internal walls, 5/16ths of an inch thick, are called shell plate.

Next, diaphragms were welded to the outside of the shell plates, connecting the cylinders with the exterior walls, or "form plate." The diaphragms radiate away from the shell plates like paddles on a Mississippi riverboat.

Once the form plate was welded on, the two half-shell modules were welded together.

Two cranes lowered the completed module into BethShip's graving dock, a 1,200-foot-long, 40-foot-deep basin where the eight modules were joined together.

Along the way, surveyors inspected the angles of mitered joints and made sure all dimensions were true. The sections had to meet exacting specifications, measurements even more precise than those used in standard shipbuilding. Each weld was visually inspected, and one in 10 was checked by ultrasound or X-ray.

By this time, all the steel was covered in a thick coating of rust, but the appearance of steel that ultimately will be surrounded by concrete and buried in mud and rock is unimportant.

Pipes and conduits that will carry electrical lines for such future amenities as lights and traffic advisory signs were installed inside the section. Reinforcing steel rods were molded around the shell plates.